/****************
*PID:
*Auth:Jonariguez
*****************
*/
#define lson k*2,l,m
#define rson k*2+1,m+1,r
#define rep(i,s,e) for(i=(s);i<=(e);i++)
#define For(j,s,e) For(j=(s);j<(e);j++)
#define sc(x) scanf("%d",&x)
#define In(x) scanf("%I64d",&x)
#define SS(x,y) scanf("d",&x,&y)
#define pf(x) printf("%d",x)
#define pfn(x) printf("%d\n",(x))
#define Pf(x) printf("%I64d",(x))
#define Pfn(x) printf("%I64d\n",(x))
#define Pc printf(" ")
#define PY puts("YES")
#define PN puts("NO")
#define MP make_pair
#define X first
#define Y second
#define PI acos(-1.0)
#include <stdio.h>
#include <string.h>
#include <string>
#include <math.h>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
typedef long long LL;
typedef int Ll;
Ll quick_pow(Ll a,Ll b,Ll MOD){a%=MOD;Ll res=1;while(b){if(b&1)res=(res*a)%MOD;b/=2;a=(a*a)%MOD;}return res;}

const int maxn=200000+10;
const int MOD=1e9+7;
vector<LL> vec1,vec2;

int main()
{
    int i,j;
    LL n,k;
    while(scanf("%I64d%I64d",&n,&k)!=EOF){
        LL s=(LL)sqrt(n);
        while(s*s<n) s++;
        vec1.clear();vec2.clear();
        for(LL i=1;i<=s;i++){
            if(i*i>n) break;
            if(n%i==0){
                vec1.push_back(i);
                if(n/i!=i)
                    vec2.push_back(n/i);
            }
        }
        sort(vec2.begin(),vec2.end());
        if(k<=vec1.size()){
            printf("%I64d\n",vec1[k-1]);
            continue;
        }
        if(k>vec1.size()+vec2.size())
            printf("-1\n");
        else printf("%I64d\n",vec2[k-vec1.size()-1]);
    }
    return 0;
}

